The present invention relates generally to photopolymerizable materials, and more specifically to polymer-dispersed liquid crystal materials suitable for recording volume holograms.
Typical state-of-the-art holographic materials do not have an electro-optical nature which can be exploited for real time control of their optical properties. That is, once the hologram is fixed, its optical characteristics cannot be changed. Thus, it is seen that there is a need for materials that can record volume holograms with properties that can be electrically controlled.
Liquid crystals have long been utilized in the prior art for their ability to change their optical orientation in the presence of an electric field. Additionally, liquid crystals can dramatically increase the diffraction efficiency of a volume hologram of which they are a part. Together, these properties offer the very desirable possibility of electrically switching the diffraction efficiency of volume holograms for use in a wide variety of optical information processing and display applications.
The prior art has attempted to combine the properties of liquid crystals with holograms by a variety of methods. Unfortunately, most of these prior art devices are complex to manufacture and are not successful at offering all the advantages of volume holographic gratings.
One approach for combining the advantages of liquid crystals with volume holographic gratings has been to first make a holographic transmission grating by exposing a photopolymerizable material with a conventional two-beam apparatus for forming interference patterns inside the material. After exposure, the material is processed to produce voids where the greatest amount of exposure occurred, that is, along the grating lines, and then, in a further step, the pores are infused with liquid crystals. Unfortunately, these materials are complex to manufacture and do not offer flexibility for in situ control over liquid crystal domain size, shape, density, or ordering.
Polymer-dispersed liquid crystals (PDLCs) are formed from a homogeneous mixture of prepolymer and liquid crystals. As the polymer cures, the liquid crystals separate out as a distinct microdroplet phase. If the polymer is a photopolymer, this phase separation occurs as the prepolymer is irradiated with light. If a photopolymerizable polymer-dispersed liquid crystal material is irradiated with light in a suitable pattern, a holographic transmission grating can be made inside the cured polymer comprising gratings of cured polymer separated by phase separated liquid crystals. The prior art has attempted to employ polymer-dispersed liquid crystal materials for writing volume gratings, but, despite a variety of approaches, has not been able to achieve high efficiency in the Bragg regime, high density (small grating spacing) capability, or low voltage ( less than 100 Vrms) switching for films in the range of 15 microns thickness. The inability to make an electrically switchable volume hologram that can be switched at voltages less than 100 volts has been a particular deficiency in the prior art in that low voltages are necessary to be compatible with conventional display and information processing technology.
It is, therefore, a principal object of the present invention to provide an improved polymer-dispersed liquid crystal system suitable for recording volume holograms.
It is a particular object of the present invention to provide a polymer-dispersed liquid crystal system that has a fast curing rate to produce small liquid crystal droplets, particularly in the range of 0.01-0.5 microns, for greater clarity of any resulting film and for writing finer gratings.
It is another object of the present invention to provide a single-step, fast holographic recording material.
It is a further object of the present invention to provide electrically switchable volume holograms that can be switched at voltages less than 100 volts.
It is also an object of the present invention to provide an improved polymer-dispersed liquid crystal system suitable for recording reflection gratings, including, in particular, switchable reflection gratings.
It is also an object of the present invention to provide an improved polymer-dispersed liquid crystal system suitable for recording subwavelength gratings, including, in particular switchable subwavelength gratings.
These and other objects of the present invention will become apparent as the description of certain representative embodiments proceeds.
The present invention provides a novel photopolymerizable material for single-step, fast recording of volume holograms with properties that can be electrically controlled. The unique discovery of the present invention is a new homogeneous mixture of a nematic liquid crystal and a multifunctional pentaacrylate monomer, with a photoinitiator, a coinitiator and a cross-linking agent, that accomplishes the objects of the invention, particularly the object of fast curing speed and small liquid crystal droplet size.
Accordingly, the present invention is directed to a polymer-dispersed liquid crystal (xe2x80x9cPDLCxe2x80x9d) material, comprising the monomer dipentaerythritol hydroxypentaacrylate, a liquid crystal, a cross-linking monomer, a coinitiator and a photoinitiator dye. The polymer-dispersed liquid crystal material may optionally further comprise a surfactant. The PDLC material may be approximately 10-40 wt % of the liquid crystal. The PDLC material may be approximately 5-15 wt % of the cross-linking monomer. The amount of the coinitiator may be 10xe2x88x923 to 10xe2x88x924 gram moles and the amount of the photoinitiator dye may be 10xe2x88x925 to 10xe2x88x926 gram moles. The surfactant, when present, may be up to approximately 6 wt % of the PDLC material.
The present invention is also directed to an electrically switchable hologram, comprising a pair of transparent plates, and sandwiched between the transparent plates, a volume hologram made by exposing an interference pattern inside a polymer-dispersed liquid crystal material, the polymer-dispersed liquid crystal material comprising, before exposure, the monomer dipentaerythritol hydroxypentaacrylate, a liquid crystal, a cross-linking monomer, a coinitiator and a photoinitiator dye. The electrically switchable hologram may optionally further comprise a surfactant.
The present invention is additionally directed to a method for reducing the switching voltage needed to switch the optical orientation of liquid crystals in a polymer-dispersed liquid crystal material, comprising the step of using alternating current switching voltage frequencies greater than 1000 Hz.
It is a feature of the present invention that a very clear and orderly separation of liquid crystal from cured polymer results, so as to produce high quality holographic transmission gratings. The prior art has achieved generally only a distribution of large and small liquid crystal domains and not the clear, orderly separation made possible by the present invention.
It is also a feature of the present invention that volume Bragg gratings with small grating spacings (approximately 4,000 lines per mm) can be recorded.
It is another feature of the present invention that in situ control of domain size, shape, density, and ordering is allowed.
It is yet another feature of the present invention that holograms can be recorded using conventional optical equipment and techniques.
It is a further feature of the present invention that a unique photopolymerizable prepolymer material is employed. This unique material can be used to record holograms in a single step.
It is also a feature of the present invention that the PDLC material has an anisotropic spatial distribution of phase-separated liquid crystal droplets within a photochemically-cured polymer matrix.
It is an advantage of the present invention that single-step recording is nearly immediate and requires no later development or further processing.
It is another advantage of the present invention that uses thereof are not limited to transmission gratings, but can be extended to other holograms such as optical storage devices and reflection and transmission pictorial holograms.
It is also an advantage that, unlike holograms made with conventional photograph-type films or dichromated gels, holograms in accordance with the present invention can be exposed in a one-step process that requires little or no further processing.
It is a further advantage of the present invention that reflection, transmission and pictorial holograms made using the teachings provided herein can be switched on and off.
It is also an advantage of the present invention that switchable reflection gratings can be formed using positive and negative dielectric anisotropy liquid crystals.
These and other features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.